A Multi-Material Flame-Retarding System Based on Expandable Graphite for Glass-Fiber-Reinforced PA6

A synergistic multi-material flame retardant system based on expandable graphite (EG), aluminum diethylphosphinate (AlPi), melamine polyphosphate (MPP), and montmorillonite (MMT) has been studied in glass-fiber-reinforced polyamide 6 (PA6). Analytical evaluations and fire performances were evaluated using coupled thermogravimetric analysis (TGA) and Fourier-transform infrared spectroscopy (FTIR) as well as cone calorimetry, UL-94 fire testing, and limiting oxygen index (LOI). A combination of EG/AlPi/MPP/MMT has been shown to provide superior flame-retarding properties when integrated at 20 wt.% into glass-fiber-reinforced PA6 (25 wt.%), achieving UL-94 V0 classification and an oxygen index of 32%. Strong residue formation resulted in low heat development overall, with a peak heat release rate (pHRR) of 103 kW/m2, a maximum of average heat release rate (MAHRE) of 33 kW/m2, and deficient total smoke production (TSP) of 3.8 m2. Particularly remarkable was the structural stability of the char residue. The char residue could easily withstand an areal weight of 35 g/cm2, showing no visible deformation.

RF Welding of Dielectric Lossless Foam Particles by the Application of a Dielectric Heatable Coating with High Recycling Potential

Due to its chemical structure and the resulting dielectric properties, the processing of the commonly used particle foam material, expanded polypropylene (ePP), is limited. Processing within the radio-frequency welding process is therefore only possible with the use of processing aids. In this paper, a new approach for the use of a solid and dielectric heatable coating for the production of three-dimensional welded components out of ePP is presented. For this purpose, three different types of water-soluble polymer polyvinyl alcohol (PVA) were analyzed as potential coating materials. The thermal and dielectric properties of the coating were further adjusted by a modification with glycerol. The maximum amount of glycerol tested was 25% by volume. It influences both the temperature development in the radio-frequency (RF) welding process as well as the adhesive bond between the ePP foam particles. It is shown that the 120 °C approach in the RF welding process resulted in a cohesive bond between the coating layers. In this way, bonded plates can be produced. In mechanical tests with compression of 20%, the manufactured plates show sufficient load capacity. Furthermore, it can be shown that a separation of PVA and ePP by type, and thereby a separation of the foam particles, is possible with the use of hot water. This might open a new way for recycling of particle foams.

Effect of Ligament Fibers on Dynamics of Synthetic, Self-Oscillating Vocal Folds in a Biomimetic Larynx Model

Synthetic silicone larynx models are essential for understanding the biomechanics of physiological and pathological vocal fold vibrations. The aim of this study is to investigate the effects of artificial ligament fibers on vocal fold vibrations in a synthetic larynx model, which is capable of replicating physiological laryngeal functions such as elongation, abduction, and adduction. A multi-layer silicone model with different mechanical properties for the musculus vocalis and the lamina propria consisting of ligament and mucosa was used. Ligament fibers of various diameters and break resistances were cast into the vocal folds and tested at different tension levels. An electromechanical setup was developed to mimic laryngeal physiology. The measurements included high-speed video recordings of vocal fold vibrations, subglottal pressure and acoustic. For the evaluation of the vibration characteristics, all measured values were evaluated and compared with parameters from ex and in vivo studies. The fundamental frequency of the synthetic larynx model was found to be approximately 200-520 Hz depending on integrated fiber types and tension levels. This range of the fundamental frequency corresponds to the reproduction of a female normal and singing voice range. The investigated voice parameters from vocal fold vibration, acoustics, and subglottal pressure were within normal value ranges from ex and in vivo studies. The integration of ligament fibers leads to an increase in the fundamental frequency with increasing airflow, while the tensioning of the ligament fibers remains constant. In addition, a tension increase in the fibers also generates a rise in the fundamental frequency delivering the physiological expectation of the dynamic behavior of vocal folds.

Investigating the Integration of Nonwoven Carbon Fibers for Mechanical Enhancement in Compression Molded Fiber-Reinforced Polymer Bipolar Plates

The demand for polymer composite solutions in bipolar plates for polymer electrolyte membrane fuel cells (PEMFCs) has risen due to advantages over metal plates such as longer lifetime, weight reduction, corrosion resistance, flexible manufacturing, freedom of design, and cost-effectiveness. The challenge with polymer composites is achieving both sufficient electrical conductivity and mechanical stability with high filler content. A carbon fiber fleece as reinforcement in a graphite-filled polypropylene (PP) matrix was investigated for use as bipolar plate material with increased mechanical and sufficient conductive properties. Plates with a thickness of 1 mm containing four layers of fleece impregnated in the PP-graphite compound were produced in a compression molding process. Particle and fiber interactions were investigated via microscopy. The plates were characterized with respect to the electrical conductivity and mechanical stability. High electric conductivity was reached for fiber-reinforced and plain PP-graphite compound plates, with increased filler content leading to a higher conductivity. The contact resistance remained largely unaffected by surface etching as no polymeric skin layer formed during compression molding. Fiber-reinforced plates exhibit twice the tensile strength, a significantly higher tensile modulus, and an increased elongation at break, compared to PP filled only with graphite.

Limitations of the Check Calculation for Tooth Deformation of Plastic Gears According to Gear Design Guideline VDI 2736

An in situ gear test rig has been developed at the Institute of Polymer Technology (LKT) to characterize the deformation of plastic gears during operation. It analyses timing differences between following index pulses of rotary encoders on the input and output shaft. This measurement principle enables the continuous measurement of the elastic tooth deformation and permanent deformations and wear at operating speed by switching between a high and low torque. Gear tests using a steel-polybutylene terephthalate (PBT) gear set were performed at different rotational speeds and tooth temperatures to analyze the tooth deformation during operation. The results were compared to the calculated deformation according to gear design guideline VDI 2736. Moreover, the results of the gear tests were correlated with the results of a dynamomechanical analysis (DMA). Both, the DMA and the in situ gear tests show that the effect of temperature on deformation is much higher than the effect of frequency or rotational speed. However, the experimentally measured tooth deformation is significantly higher (up to 50%) than the calculated at lower speed. Thus, the check calculation according to VDI 2736 underestimates the actual tooth deformation at lower speeds. Therefore, the guideline should be adjusted in the future.

Test Rig for The In Situ Measurement of The Elastic Tooth Deflection of Plastic Gears

A new steel–plastic gear set testing methodology has been developed at the Institute of Polymer Technology (LKT). The in situ gear test rig analyses the timing differences between the index pulses of rotary encoders on the input and output shaft. This measurement principle enables the continuous measurement of the elastic tooth deflection on the one hand and permanent deformations and wear on the other hand by switching between a high loading torque and a low measuring torque. However, the elastic tooth deflection measurement using this principle has not yet been validated. Therefore, in situ gear tests using polybutylene terephthalate (PBT) gears were performed to evaluate the elastic tooth deflection of the plastic gear during operation. The results were compared to the results of pulsator tests. The comparison shows a very good correlation between the results of the newly developed in situ gear test rig and the well-established pulsator test rig. However, it has been shown that the test rig design creates a measuring offset due to angular displacements of the shafts due to torsion of test rig components.

Electrodeposition model with dynamic ion diffusion coefficients for predicting void defects in electroformed microcolumn arrays

Due to the confined mass transfer capability in microchannels, void defects are easily formed in electroformed microcolumn arrays with a high depth/width ratio, which seriously affects the life and performance of micro-devices. The width of the microchannel constantly decreases during electrodeposition, which further deteriorates the mass transfer capability inside the microchannel at the cathode. In the traditional micro-electroforming simulation model, the change of the ion diffusion coefficient is always ignored, making it difficult to accurately predict the size of void defects prior to electroforming experiments. In this study, nickel ion diffusion coefficients in microchannels are tested based on the electrochemical experiments. The measured diffusion coefficients decrease from 4.74 × 10^-9 to 1.27 × 10^-9 m² s^-1, corresponding to microchannels with a width from 120 to 24 μm. The simulation models of both constant and dynamic diffusion coefficients are established, and the corresponding simulation results are compared with the void defects obtained using micro-electroforming experiments. The results show that when the cathode current densities are 1, 2 and 4 A dm^-2, the size of void defects obtained with the dynamic diffusion coefficient model is closer to the experimental results. In the dynamic diffusion coefficient model, the local current density and ion concentration distribution proves to be more inhomogeneous, leading to a big difference in the deposition rate of nickel between the bottom and the opening of a microchannel, and consequently a larger void defect in the electroformed microcolumn arrays. In brief, the ion diffusion coefficient inside microchannels with a different width is tested experimentally, which provides a reference for developing reliable micro-electroforming simulation models.

Eutectic In Situ Modification of Polyamide 12 Processed through Laser-Based Powder Bed Fusion

Laser-based powder bed fusion (LPBF) of polymers allows for the additive manufacturing of dense components with high mechanical properties. Due to inherent limitations of present material systems suitable for LPBF of polymers and required high processing temperatures, the present paper investigates the in situ modification of material systems using powder blending of p-aminobenzoic acid and aliphatic polyamide 12, followed by subsequent laser-based additive manufacturing. Prepared powder blends exhibit a considerable reduction of required processing temperatures dependent on the fraction of p-aminobenzoic acid, allowing for the processing of polyamide 12 at a build chamber temperature of 141.5 °C. An elevated fraction of 20 wt% of p-aminobenzoic acid allows for obtaining a considerably increased elongation at break of 24.65% ± 2.87 while exhibiting a reduced ultimate tensile strength. Thermal investigations demonstrate the influence of the thermal material history on thermal properties, associated with the suppression of low-melting crystalline fractions, yielding amorphous material properties of the previously semi-crystalline polymer. Based on complementary infrared spectroscopic analysis, the increased presence of secondary amides can be observed, indicating the influence of both covalently bound aromatic groups and hydrogen-bound supramolecular structures on emerging material properties. The presented approach represents a novel methodology for the energy-efficient in situ preparation of eutectic polyamides, potentially allowing for the manufacturing of tailored material systems with adapted thermal, chemical, and mechanical properties.

Influence of production process-induced surface topologies at varying roughness depths on the tribological properties of polyamide steel contact

Friction and wear in a tribological system are directly dependent on the surface structure and roughness of the friction partners involved. In this article, a clear interaction between surface topologies and their roughness depth was identified for the material pairing polyamide 66 – steel. The typical correlation between roughness and wear, initially decreasing and increasing after a wear minimizing roughness, was found for all surface topologies, albeit at different levels. The effect of the surface topology is negligible at low roughness (S z  < 2.0 µm) with adhesive wear processes determining the wear behaviour. At higher roughness depths (S z  > 2.0 µm), the ability of the surface topology to form a stable transfer film determines the tribological behaviour by limiting the effect of abrasive wear processes. A stable transfer film is formed with sufficient roughness and undercuts in the direction of motion, which can be characterised by the average roughness depth, R z , in the direction of motion. Based on these empirical results, an explanatory model for the observed behaviour is presented.

Possibilities of Integrated Fabrication of Insulation Systems in Electric Drives by Injection Molding of Thermosets

Due to the increasing demand for electro mobility and specifically for electrified vehicles, the demand for electric drive technology is expanding significantly with changing requirements in terms of the process and the application. The electrical insulation system of the stator is an essential part of the fabrication process with a high impact on the application properties. Due to limitations-for example, in terms of suitable materials for the stator insulation-a new technology of integrated fabrication by injection molding of thermosets has been founded. In this study, two epoxy (EP) types with different fillers were investigated to prove their suitability in terms of the material properties in the fabrication process and the application. A general realization of the integrated fabrication of insulation systems in electrical engineering by injection molding was proved. Further, the differences regarding the suitability of the two materials are portrayed. It was demonstrated that mainly the filler material influences the fabrication process and the properties in the application, leading to differing suitability in terms of the EP 3162 EMG within the fabrication process and in terms of XW 6640-1 within the application properties of the thermal conductivity and the thermal linear expansion. It was further shown that the filler within the material system is required to increase the thermal conductivity needed for the application. The inclusion of the filler influences the reaction kinetics and the viscosity behavior. A fabrication of the material with fillers is however still possible.

Reducing component stress during encapsulation of electronics: a simulative examination of thermoplastic foam injection molding

The direct encapsulation of electronic components is an effective way of protecting components against external influences. In addition to achieving a sufficient protective effect, there are two other big challenges for satisfying the increasing demand for encapsulated circuit boards. The encapsulation process should be both suitable for mass production and offer a low component load. Injection molding is a method with good suitability for large series production but also with typically high component stress. In this article, two aims were pursued: first, the development of a calculation model that allows an estimation of the occurring forces based on process variables and material parameters. Second, the evaluation of a new approach for stress reduction by means of thermoplastic foam injection molding. For this purpose, simulation-based process data was generated with the Moldflow simulation tool. Based on this, component stresses were calculated with the calculation model. The suitability of the new approach was clearly demonstrated and a significant reduction in shear forces during overmolding was achieved. It was possible to demonstrate a process development that makes it possible to meet the two main requirements of direct encapsulation in addition to a high protective effect.

In-situ leakage behavior of polymer-metal hybrids under mechanical load

Tightness against media is a frequent requirement for technical components. Despite various standardized test procedures, failure regularly occurs during use. Often, no clear cause for failure can be determined afterwards. In this article, a new test setup is presented and applied, which allows an in-situ measurement during a mechanical load. A flow meter with a measuring range of 0.02 mL/min to 5 mL/min is used for this purpose. This makes it possible to determine leakage rates with time resolution and thus to identify the moment of failure or the causal failure load. This new method was applied directly to different aluminum inserts with a polyamide 66 (PA66) overmold. It was shown that no increase in leakage occurs until a maximum force is reached, even with multiple loads. This maximum force depends only on the pretreatment of the inserts and can be determined in a simple pull-out test independently of the test setup used here and can therefore be used for the design of assemblies. In the test, a maximum force of 100 N was achieved for untreated inserts, 140 N for adhesion promoter-coated parts and 600 N for etched inserts with a contact area of 48 mm2. With this results, a new link between adhesion and tightness can be shown, which of course is only valid for initial tight parts.

The Impact of β-Radiation Crosslinking on Flammability Properties of PA6 Modified by Commercially Available Flame-Retardant Additives

A comparative study was conducted investigating the influence of β-radiation crosslinking (β-RC) on the fire behavior of flame retardant-modified polyamide 6 (PA6). In order to provide a comprehensive overview, a variety of commercially available flame-retardant additives were investigated, exhibiting different flame retarding actions such as delusion, char formation, intumescence and flame poisoning. This study focused on the identification of differences in the influence of β-RC on fire behavior. Coupled thermal gravimetrical analysis (TGA) and Fourier transformation infrared spectroscopy (FTIR) were used to conduct changes within the decomposition processes. Dynamic thermal analysis (DTA) was used to identify structural stability limits and fire testing was conducted using the limiting oxygen index (LOI), vertical UL-94 and cone calorimeter testing. Crosslinking was found to substantially change the fire behavior observed, whereas the observed phenomena were exclusively physical for the given formulations studied: warpage, char residue destruction and anti-dripping. Despite these phenomena being observed for all β-RC formulations, the impact on fire resistivity properties were found to be very different. However, the overall fire protection properties measured in UL-94 fire tests were found to deteriorate for β-RC formulations. Only β-RC formulations based on PA6/EG were found to achieve a UL-94 V0 classification.

Effects of Fumed Silica on Thixotropic Behavior and Processing Window by UV-Assisted Direct Ink Writing

In this research, the effects of fumed silica (FS) on the Ultraviolet (UV)-ink rheological behavior and processing windows were discussed. Objects using different concentrations of FS inks were printed by the modified UV-Direct ink writing (DIW) printer. The function of fumed silica in the ink-based system has been verified, and the processing scope has been expended with a suitable amount of FS combined with the UV light. The results show that the combination of a suitable amount of FS with the UV-DIW system reaches fast and accurate printing with a larger processing window compared to the non-UV system. However, an excessively high concentration of FS will increase the yield stress of the ink, which also increases the requirement of extrusion unit and the die-swelling effects.

Filling Behavior in Joining Using Pin-like Structures

Multi-material designs enable more efficient use of material-specific properties, which is necessary for sustainable and resource-saving production. However, multi-material polymer joints confront conventional joining methods with major challenges. Therefore, novel joining processes such as joining using pin-like structures are required. Investigations into this innovative process have provided initial findings of, for example, the design criteria of the pin-like structures depending on the material combination. For further optimization of the process, the filling behavior and the shrinkage effects occurring in pin-like joining are herein investigated. These have a decisive influence on the resulting bond quality. To identify the correlations, the joining step was carried out on the one hand using vibration welding technology with and without pre-heating of the structured-partner. On the other hand, the injection molding process was used to realize filling of the structures, as well as cooling under increased pressure. The investigations show that the shrinkage behavior clearly influences the filling degree and the bond properties of the multi-material joint. For shrinkage-intensive materials, filling and cooling under pressure is essential to achieve high mechanical bond strengths, whereas for materials with low shrinkage, the pressure during the joining step is negligible.

Expandable Graphite as a Multifunctional Flame-Retarding Additive for Highly Filled Thermal Conductive Polymer Formulations

Expandable graphite (EG) and graphite (G) were assessed as multifunctional additives improving both flame retardancy and thermal conductivity in highly filled, thermal conductive polymeric materials based on polyamide 6 (PA6). Fire testing was conducted using modern UL-94, LOI and cone calorimeter test setups. It is demonstrated that thermal conductivity can significantly influence the time to ignition, although offering little fire resistance once ignited even in highly filled systems. Thus, for PA6 formulations containing solely 70 wt.% G, the peak heat release rate (pHRR) measured in cone calorimeter tests was 193 kW/m², whereas PA6 formulations containing 20 wt.% EG/50 wt.% G did not exhibit a measurable heat development. Particular attention was paid to effect separation between thermal conductivity and residue formation. Good thermal conductivity properties are proven to be particularly effective in test scenarios where the heat impact is comparatively low and the testing environment provides good heat dissipation and convective cooling possibilities. For candle-like ignition scenarios (e.g., LOI), filling levels of >50 wt.% (G or EG/G) are shown to be sufficient to suppress ignition exclusively by thermal conductivity. V0 classifications in UL-94 vertical burning tests were achieved for PA6 formulations containing ≥70 wt.% G, ≥25 wt.% EG and ≥20 wt.% EG/25 wt.% G.

Two Layer Sheets for Processing Post-Consumer Materials

An increasing percentage of post-consumer materials (PCR) is becoming more and more important in all processing methods in polymer technology, also due to the lack of raw materials and political demands. Very special requirements are placed on material properties such as viscosities in extrusion. Low viscosities and the presence of particles affect extrusion in a negative manner. In this study, the use of multilayer sheets is determined to both ensure extrudability and contribute to a significant improvement in surface qualities. The focus is placed on the influence of viscosity and particles on mono- und multilayer sheet quality. Therefore, two different virgin materials with a melt flow rate (MFR) of 3 g/10 min and 6 g/10 min and two different PCR materials with a MFR of 16 g/10 min and 50 g/10 min are processed both in monolayers and in two layer sheets. Rheological investigations, optical analysis, and film thickness distributions are used to show the relationship between matrix viscosity and particles. The results show that the use of multilayer extrusion can improve both extrudability and sheet quality, so that multilayer sheets can offer a significant potential in the processing of PCR materials.

Inline Quality Control through Optical Deep Learning-Based Porosity Determination for Powder Bed Fusion of Polymers

Powder bed fusion of thermoplastic polymers is a powder based additive manufacturing process that allows for manufacturing individualized components with high geometric freedom. Despite achieving higher mechanical properties compared to other additive manufacturing processes, statistical variations in part properties and the occurrence of defects cannot be avoided systematically. In this paper, a novel method for the inline assessment of part porosity is proposed in order to detect and to compensate for inherent limitations in the reproducibility of manufactured parts. The proposed approach is based on monitoring the parameter-specific decay of the optical melt pool radiance during the melting process, influenced by a time dependency of optical scattering within the melt pool. The underlying methodology compromises the regression of the time-dependent optical melt pool properties, assessed in visible light using conventional camera technology, and the resulting part properties by means of artificial neural networks. By applying deep residual neural networks for correlating time-resolved optical process properties and the corresponding part porosity, an inline assessment of the spatially resolved part porosity can be achieved. The authors demonstrate the suitability of the proposed approach for the inline porosity assessment of varying part geometries, processing parameters, and material aging states, using Polyamide 12. Consequently, the approach represents a methodological foundation for novel monitoring solutions, the enhanced understanding of parameter-material interactions and the inline-development of novel material systems in powder bed fusion of polymers.

Rapid Enrichment of Submicron Particles within a Spinning Droplet Driven by a Unidirectional Acoustic Transducer

Efficient and rapid particle enrichment at the submicron scale is essential for research in biomedicine and biochemistry. Here, we demonstrate an acoustofluidic method for submicron particle enrichment within a spinning droplet driven by a unidirectional transducer. The unidirectional transducer generates intense sound energy with relatively low attenuation. Droplets placed offset in the wave propagation path on a polydimethylsiloxane film undergo strong pressure gradients, deforming into an ellipsoid shape and spinning at high speed. Benefitting from the drag force induced by the droplet spin and acoustic streaming and the radial force induced by the droplet compression and expansion, the submicron particles in the liquid droplet quickly enrich toward the central area following a spiral trajectory. Through numerical calculations and experimental processes, we have demonstrated the possible mechanism responsible for particle enrichment. The application of biological sample processing has also been exploited. This study anticipates that the strategy based on the spinning droplet and particle enrichment method will be highly desirable for many applications.

A Dyciandiamine-Based Methacrylate-Epoxy Dual-Cure Blend-System for Stereolithography

In this research, an epoxy-based dual-cure system is developed and characterized for SLA additive manufacturing. Dual-cure systems consist of UV-curable acrylates and thermal active components. The second curing step offers an additional degree of freedom to design specific material properties. In this study, a blend of varying concentrations of an epoxy/curing agent mix, respectively, DGEBA, DICY and photocurable methacrylate, was used to create a material that is printable in the SLA process into a UV-cured or green part and subsequently thermally cured to achieve superior thermal and mechanical properties. Calorimetric measurements were performed to determine the reactivity of the thermal reaction at different concentrations of epoxy. The fully cured specimens were tested in mechanical and dynamic mechanical measurements, and the results showed a significant improvement in tensile stress and glass transition temperature with rising epoxy concentrations. Fractured surfaces from tensile testing were investigated to further characterize the failure of tested samples, and thermal degradation was determined in TGA measurements, which showed no significant changes with an increasing epoxy concentration.

Process Behavior of Short Glass Fiber Filled Systems during Powder Bed Fusion and Its Effect on Part Dimensions

In powder bed fusion of polymers, filled systems can provide a wide range of part properties, which is still a deficit in additive manufacturing, as the material variety is limited. Glass fiber filled polymers provide a higher strength and stiffness in parts; nevertheless, the process behavior differs from neat polymer systems. In this study, the optical properties and their effect on the part dimensions are analyzed. A higher glass fiber content leads to an increased absorption of laser energy, while the specific heat capacity decreases. This results in larger part dimensions due to higher energy input into the powder bed. The aim of the study is to gain process understanding in terms of ongoing mechanisms during processing filled systems on the one hand and to derive strategies for filled polymer systems in powder bed fusion on the other hand.

Model Approach for Displaying Dynamic Filament Displacement during Impregnation of Continuous Fibres Based on the Theory of Similarity – Theory and Modelling

The underlying process for the production of textile reinforced thermoplastics is the impregnation of dry textile reinforcements with a thermoplastic matrix. The process parameters such as temperature, time and pressure of the impregnation are mainly determined by the permeability of the reinforcement. This results from a complex interaction of hydrodynamic compaction and relaxation behavior caused by textile and process parameters. The foundation for the description and optimization of impregnation progresses is therefore the determination of the pressure-dependent permeability of fibre textiles. Previous experimental investigations have shown that the dynamic compaction behavior during the impregnation of fibre reinforcements with thermoplastics or thermosets can be successfully characterized. However, for most cases, an analytical representation has not been possible due to the complexity of the process. Although it may be possible to reproduce this behavior by numerical calculations, the results need to be confirmed by experiments. This paper lays the analytical foundation for building a scaled model system, based on the theory of similarity, to observe, measure, and evaluate the dynamic compaction behavior of textile reinforcements under controlled process conditions.

Design criteria for the pin-foot ratio for joining adhesion-incompatible polymers using pin-like structures in vibration welding process

Joining technologies have a crucial role in the product development process, e.g. to achieve local part properties or functional integrations. This often requires multi-material joints, which are challenging for conventional joining processes. Therefore, innovative processes are needed to generate bonds between adhesion-incompatible material combinations, such as joining using pin-like structures in the vibration welding process. Investigations into this novel process have provided initial findings; however, a specific pin design is not possible at this time. For this reason, the influence of the pin-foot width of the two joining partners was analyzed numerically by simulation. The results of the simulation were validated by experimental tests. The investigations show, that the simulation model is suitable for predicting the bond quality as well as the fracture behavior of the multi-material joint based on pin-like structures. The developed correlations between material, pin-like structure, and resulting bond quality allow design criteria for the pin-like structures to be derived. These allow a specific dimensioning of the pin-foot ratio depending on the used material combination. Thus, for example, the fracture behavior of the multi-material connection can be selectively adjusted, as well as the bond strength can be maximally utilized.

Achieving a 3D Thermally Conductive while Electrically Insulating Network in Polybenzoxazine with a Novel Hybrid Filler Composed of Boron Nitride and Carbon Nanotubes

To solve the problem of excessive heat accumulation in the electronic packaging field, a novel series of hybrid filler (BN@CNT) with a hierarchical “line-plane” structure was assembled via a condensation reaction between functional boron nitride(f-BN) and acid treated carbon nanotubes (a-CNTs). The reactions with different mass ratios of BN and CNTs and the effect of the obtained hybrid filler on the composites’ thermal conductivity were studied. According to the results, BN@15CNT exhibited better effects on promoting thermal conductivity of polybenzoxazine(PBz) composites which were prepared via ball milling and hot compression. The thermally conductive coefficient value of PBz composites, which were loaded with 25 wt% of BN@15CNT hybrid fillers, reached 0.794 W· m⁻¹· K⁻¹. The coefficient value was improved to 0.865 W· m⁻¹· K⁻¹ with 15 wt% of BN@15CNT and 10 wt% of BN. Although CNTs were adopted, the PBz composites maintained insulation. Dielectric properties and thermal stability of the composites were also studied. In addition, different thermal conduction models were used to manifest the mechanism of BN@CNT hybrid fillers in enhancing thermal conductivity of PBz composites.

Flame-Retardant Polyamide Powder for Laser Sintering: Powder Characterization, Processing Behavior and Component Properties

Up to now, laser-sintered components have been barely used by industries such as aerospace and transport industry due to high flammability. By the use of flame retardants, the flammability of laser-sintered parts should be reduced to extend their range of possible applications. This paper aims to investigate the influence of halogen-free phosphinate-based flame retardants on process-relevant characteristics and process behavior, as well as mechanical and physical properties. Most importantly, the flammability of the material should be reduced. Two different types of phosphinate-based fillers were used in a concentration between 10 and 25 wt % in combination with the matrix material polyamide 12 (PA12). Thermal, optical, and powder properties of the mixtures were analytically investigated. Furthermore, the mechanical characterization of the sintered specimen was carried out. The addition of filler in laser sintering changes the process behavior and properties of the component. With this investigation, the correlation among flame retardants, process-relevant characteristics, process behavior, and resulting part properties was derived for the first time. Finally, a mixture of 15-20 wt % of flame retardant leads to the best trade-off between flame retardancy and mechanical properties.

Particle Size Related Effects of Multi-Component Flame-Retardant Systems in Poly(Butadiene Terephthalate)

Aluminum tris-(diethylphosphinate) (AlPi) is known to have an efficient flame-retardant effect when used in poly(butadiene terephthalates) (PBT). Additionally, better flame-retardant effects can be achieved through the partial substitution of AlPi by boehmite in multi-component systems, which have been shown to be an effective synergist due to cooling effects and residue formation. Although the potential of beneficial effects is generally well known, the influence of particle sizes and behavior in synergistic compositions are still unknown. Within this paper, it is shown that the synergistic effects in flammability measured by limiting oxygen index (LOI) can vary depending on the particle size distribution used in PBT. In conducting thermogravimetric analysis (TGA) measurements, it was observed that smaller boehmite particles result in slightly increased char yields, most probably due to increased reactivity of the metal oxides formed, and they react slightly earlier than larger boehmite particles. This leads to an earlier release of water into the system enhancing the hydrolysis of PBT. Supported by Fourier transformation infrared spectroscopy (FTIR), we propose that the later reactions of the larger boehmite particles decrease the portion of highly flammable tetrahydrofuran in the gas phase within early burning stages. Therefore, the LOI index increased by 4 vol.% when lager boehmite particles were used for the synergistic mixture.

Curing Kinetic Analysis of Acrylate Photopolymer for Additive Manufacturing by Photo-DSC

In this research, the curing degree of an acrylate-based monomer using direct UV-assisted writing technology was characterized by differential photo calorimetry (Photo-DSC) to investigate the curing behavior. Triggered by the UV light, the duo function group monomer 1,6-Hexamethylene diacrylate (HDDA), photoinitiator 1173 and photoinhibitor exhibit a fast curing process. The exothermal photopolymerization reaction was performed in the isothermal mode in order to evaluate the different thermal effects that occurred during the photopolymerization process. The influences of both UV light intensity and exposure time were studied with single-factor analysis. The results obtained by photo-DSC also allow us to perform the kinetic study of the polymerization process: The results show that, for the reaction, the higher the UV intensity, the higher the curing degree together with faster curing speed. At the same time, the effect of the heat released during the exothermic reaction is negligible for the polymerization process. When increasing the exposure time, limited improvement of curing degree was shown, and the distribution is between 65-75%. The reaction enthalpy and related curing degree work as a function of time. The Avrami theory of phase change was introduced to describe the experimental data. The functions of a curing degree with light intensity and exposure time were achieved, respectively.

Air inclusions in the polymer melt functioning as intrinsic physical blowing agents for the generation of foams in rotational molding

In recent years, foams have experienced a major economic uprise, not least due to their lightweight construction potential. In this article, a new process variation is presented, which enables the generation of foamed structures in rotational molding by the utilization of vacuum. The novel method is based on entrapped air in the melt as an intrinsic physical blowing agent. By applying negative pressure in the cooling or solidification phase, the air bubbles expand. The crystallization freezes the existing conditions and thus forms the foamed structure. The investigations presented consider influences by different pressures as well as the temperature at which the vacuum is applied. The results with polyethylene show that by varying the pressure as well as the application temperature of the vacuum, components with different densities and cell characteristics result. The resulting foamed components excel by an improved stiffness per unit weight ratio.

The influence of mold temperature on thermoset in-mold forming

A new process, called thermoset in-mold forming, for combining thermoset master forming and thermoset forming in one mold is in development. A pre-impregnated continuous-fiber reinforced sheet based on epoxy (prepreg) is formed in the injection molding machine, followed by instantaneous overmolding of a short-fiber reinforced epoxy compound in one step. Compared with conventional processes in which thermoset injection molding, prepreg compression molding, and hence curing of the materials are separated, the new process allows for the combination in one step and simultaneous curing of both components. The result is a hybrid component, which features a continuous-fiber reinforced part for higher mechanical performance and a short-fiber reinforced part with high design freedom for integration of additional functions. For a successful combination of both materials in one process, it is essential to investigate the bond strength between them in relation to the processing parameters and their influence on the degree of cure. This paper analyzes the influence of the mold temperature in this process on curing degree, bond strength, and the processing viscosity.

The Effect of Temperature and Strain Rate on the Interfacial Behavior of Glass Fiber Reinforced Polypropylene Composites: A Molecular Dynamics Study

To make better use of fiber reinforced polymer composites in automotive applications, a clearer knowledge of its interfacial properties under dynamic and thermal loadings is necessary. In the present study, the interfacial behavior of glass fiber reinforced polypropylene (PP) composites under different loading temperatures and strain rates were investigated via molecular dynamics simulation. The simulation results reveal that PP molecules move easily to fit tensile deformation at higher temperatures, resulting in a lower interfacial strength of glass fiber-PP interface. The interfacial strength is enhanced with increasing strain rate because the atoms do not have enough time to relax at higher strain rates. In addition, the non-bonded interaction energy plays a crucial role during the tensile deformation of composites. The damage evolution of glass fiber-PP interface follows Weibull's distribution. At elevated temperatures, tensile loading is more likely to cause cohesive failure because the mechanical property of PP is lower than that of the glass fiber-PP interface. However, at higher strain rates, the primary failure mode is interfacial failure because the strain rate dependency of PP is more pronounced than that of the glass fiber-PP interface. The relationship between the failure modes and loading conditions obtained by molecular dynamics simulation is consistent with the author's previous experimental studies.

Influence of chemical postprocessing on mechanical properties of laser-sintered polyamide 12 parts

A limiting factor for industrial usage of laser-sintered parts is the high surface roughness due to the semi-molten or attaching powder particles resulting from tool and pressureless manufacturing. An approach to improve the surface quality is the postprocessing with acids to smoothen the surface as it enables improvement without geometrical restrictions of the parts. The present work deals with the usage of nitric, hydrochloric, and trifluoroacetic acids, and exhibits the influence on the resulting surface morphology, dimensional accuracy, and the mechanical properties. The results exhibit different interaction mechanics and show great differences in the resulting part properties.

Influence of the Mold Temperature and Part Thickness on the Replication Quality and Molecular Orientation in Compression Injection Molding of Polystyrene

It is well known that the process of injection molding with dynamic mold temperature control leads to a good replication quality of high aspect ratio microstructures. However, the inhomogeneous pressure distribution during the holding pressure phase results in an anisotropy of the component properties, low dimensional accuracy and, especially with optical polymers, in undesired stress birefringence. The anisotropy is based on the orientation of the molecular chains in the flow direction, which can be reduced by an injection-compression molding (ICM) process. In order to use the synergy from both processes, an injection-compression molding process with dynamic mold temperature control can be utilized. Within the scope of this investigation, the new process was reproduced by an ICM process at elevated mold temperature (ICM_EMT) and compared with injection molding (IM) with regard to molding accuracy and optical properties in dependence of component thickness and mold temperature. In order to evaluate the molding accuracy, the roughness of a wire-eroded microstructure on the cavity surface was measured. To determine the degree of orientation, the optical properties considered were the transmission and the path difference. It was shown that the adapted ICM process was able to achieve a high degree of replication accuracy with a low degree of orientation, especially for thin-walled components. ICM at elevated mold temperature reduced the path difference in the components with the lowest wall thickness by a factor of two while at the same time optimizing the replication of the microstructure. This could also be confirmed by transmission measurements.

Influence of titanium oxide-based colourants on the morphological and tribomechanical properties of injection-moulded polyoxymethylene spur gears

Regardless of colouration for functional or aesthetic purposes, technical polymer parts, like gears, require consistent properties. However, there is a lack of research into the effect of colourants on the tribomechanical properties of gears. Therefore, the effects of two pigments, titanium dioxide (white) and chrome antimony titanium oxide (yellow), and three delivery methods, masterbatch, liquid colour and direct compounding, on part morphology, dimensions, tribological and mechanical performance of injection-moulded polyoxymethylene (POM) spur gears are investigated in this paper. The white pigment accelerates the crystallisation of POM, causing fine and highly-crystalline morphological structures and smaller dimensions. However, the yellow pigment decelerates crystallisation, resulting in a coarser morphology with highly crystalline core material and bigger parts. Furthermore, the delivery method affects only the tribomechanical properties. Using a masterbatch decreases loads at break and increases deflection at break, since the carrier material acts as an impact modifier and a weak spot. The liquid colour decreases wear due to lubricating properties, whereas the pure pigments increases abrasion, especially in combination with a coarse microstructure. However, the effects of carrier systems and changes in morphology are always superimposed. Considering the performance and tolerance of technical components, colourants have to be carefully selected to ensure beneficial properties.

Influence of radiation-crosslinking on the elongation behaviour of glass-fibre-filled sheets in the thermoforming process

Thermoforming is one of the most important processes in polymer processing. In the packaging industry, thermoformed parts such as blister packs are manufactured from amorphous plastics such as polystyrene (PS) or polyvinyl chloride (PVC). In the field of semi-crystalline thermoplastics, mainly standard thermoplastics, such as, for example, polypropylene (PP), polyethylene (PE), or polyethylene terephthalate (PET), are used. There is limited literature dealing with the thermoforming of thin filled systems. Filler bonding, in particular, represents a major challenge in strain rheology. Electron irradiation is a way to generate improved filler-matrix bonding. In this study, the influence of fillers and radiation-crosslinking on the elongation behaviour and on the wall thickness distribution was investigated. At higher areal draw ratios, an enormous benefit of radiation-crosslinking of thin filled sheets is shown. While non-crosslinked specimens could not be formed, it was possible to thermoform radiation-crosslinked sheets filled with 10 vol.% glass fibres. Furthermore, with the higher areal draw ratio, the influence of the filler orientation on the stretching behaviour became more apparent.

Processing LDS-Circuit Boards by Selective Laser Sintering

As the demand for individualized products rises, the development and need for additive manufacturing (AM) techniques such as selective laser sintering (SLS) has strongly increased. The industrial use of these procedures for prototypes or small-scale production lines has grown due to their specific characteristics like the high achievable complexity. With the increasing demand for electrification and functionalization, the combination of AM with laser-direct structuring (LDS) gains interest. Therefore, the powder used for the investigation is dry coated with a LDS-additive, which enables laser activation and a metallization of the activated sections in a metallization bath. To characterize the influence of the LDS-additive on the process, the powder properties were investigated for unfilled and successive increased additive content. The thermal process window was identified by standard and process adapted isothermal differential scanning calorimetry. This showed a decrease of the isothermal crystallization time due to nucleation effects of the additive. Subsequently, parts were produced with a parameter study and showed a demand for a higher energy density. The resulting parts were then metallized with a parameter variation and characterized by stereomicroscopy. To investigate the influence of the different parameter sets and the LDS content, the mechanical properties were determined.

Characterization of Microchannel Replicability of Injection Molded Electrophoresis Microfluidic Chips

Microfluidic chips have been widely applied in biochemical analysis, DNA sequencing, and disease diagnosis due to their advantages of miniaturization, low consumption, rapid analysis, and automation. Injection molded microfluidic chips have attracted great attention because of their short processing time, low cost, and mass production. The microchannel is the critical element of a microfluidic chip, and thus the microchannel replicability directly affects the performance of the microfluidic chip. In the current paper, a new method is proposed to evaluate the replicability of the microchannel profile via the root mean square value of the actual profile curve and the ideal profile curve of the microchannel. To investigate the effects of injection molding parameters (i.e., mold temperature, melting temperature, holding pressure, holding time, and injection rate) on microchannel replicability, a series of single-factor experiments were carried out. The results showed that, within the investigated experimental range, the increase of mold temperature, melt temperature, holding pressure, holding time, and injection rate could improve microchannel replicability accuracy. Specifically, the microchannels along the flow direction of the polymer melt were significantly affected by the mold temperature and melt temperature. Moreover, the replicability of the microchannel was influenced by the distance from the injection gate. The effect of microchannel replication on electrophoresis was demonstrated by a protein electrophoresis experiment.

Spherical Polybutylene Terephthalate (PBT)-Polycarbonate (PC) Blend Particles by Mechanical Alloying and Thermal Rounding

In this study, the feasibility of co-grinding and the subsequent thermal rounding to produce spherical polymer blend particles for selective laser sintering (SLS) is demonstrated for polybutylene terephthalate (PBT) and polycarbonate (PC). The polymers are jointly comminuted in a planetary ball mill, and the obtained product particles are rounded in a heated downer reactor. The size distribution of PBT⁻PC composite particles is characterized with laser diffraction particle sizing, while the shape and morphology are investigated via scanning electron microscopy (SEM). A thorough investigation and characterization of the polymer intermixing in single particles is achieved via staining techniques and Raman microscopy. Furthermore, polarized light microscopy on thin film cuts enables the visualization of polymer mixing inside the particles. Trans-esterification between PBT and PC during the process steps is investigated via vibrational spectroscopy and differential scanning calorimetry (DSC). In this way, a new process route for the production of novel polymer blend particle systems for SLS is developed and carefully analyzed.

Determination of the fundamental dimension development in building direction for laser-sintered parts

The additive manufacturing process of the laser sintering of polymers (LS) allows the production of complex parts right from CAD data. However, the manufactured parts often show dimensional inaccuracies. In order to fundamentally determine the influencing parameters on the accuracy of LS parts, a hatching specimen, a layer-specimen and defined part geometries are manufactured and subsequently measured. These, combined with a theoretical observation of the layer wise geometry buildup, are used to determine the fundamental portions of the development of dimensions in building direction (z-direction). The results indicated a defined powder adhesion height at the top and the bottom of a melted layer, along with the dependency of melt depth and the hatch number for small structures. Depending on the nominal heights of an LS part, either an oversize or undersize was detected.